Populations of F. lycioides Andrews are composed of small-flowered female plants and an equal number of larger-flowered hermaphrodites, which may be female fertile, or morphologically or physiologically female sterile. A major selective force driving the evolution of separate sexes in F. lycioides is suggested to be the partitioning of limited resources associated with pollinator maintenance and fruit production in a semi-arid and mediterranean climate. The hummingbird Rhodopis vesper atacamensis is the only known pollinator, and appears to be energetically dependent upon F. lycioides. Hermaphrodites are facultative in their fruit production, produce as much as 6 times more nectar than females, and feed birds both prior to flower opening and during anthesis. Females bear abundant fruit and produce only a relatively small amount of nectar during anthesis. Nectar production is extremely variable in both sexes but is unpredictable in hermaphrodites and apparently predictable in females. The unpredictability of hermaphrodite nectar production may be a key factor permitting the evolution of resource partitioning into large-flowered bird-feeding pollen plants and small-flowered reproductive individuals.